Cellulose bleach tower with means for spreading a bleaching agent therein

ABSTRACT

A cellulose bleach tower in which at least two different bleaching steps are performed upon pulp flowing continuously therethrough, is provided with a set of concentric cylindrical screens through which a liquid containing used bleaching chemicals is drawn off the pulp at the end of the first bleaching step. A liquid containing the bleaching chemical of the second step is spread by nozzles moving between or laterally of the screens. After the pulp has passed the screens and before the main part of the second bleaching step has taken place, the concentration of the bleaching chemical is equalized over the cross section of the tower by means of rotating scrapers which in a short zone of the tower displace the pulp back and forth in the transverse direction of the tower, whereas the pulp else flows truly axially of the tower.

United States Patent [72] Inventor Johan C. F. C. Richter St. Jean Cap Ferrat, France [21] Appl. No. 773,385 [22] Filed Nov. 5, 1968 [45] Patented Aug. 17, 1971 [73] Assignee Aktiebolaget Kamyr Karlstad, Sweden [32] Priority Nov. 6, 1967 I 3 3 I Sweden [31] 15150/1967 [54] CELLULOSE BLEACH TOWER WITH MEANS FOR SPREADING A BLEACHING AGENT THEREIN 4 Claims, 3 Drawing Figs.

[52] U.S.Cl 68/181 R, 68/207 [51] Int. Cl D2Jld 5/02 [50] Field of Search 8/156; 68/181, 182, 207

[56] References Cited UNITED STATES PATENTS 2,431,478 11/1947 Hill 68/181 X 3,298,209 1/1967 Laakso 68/181 3,348,390 10/1967 Richter.. 68/181 3,398,412 8/1968 Aremaa 8/156 Primary Examiner-William T. Price Attorney-Cushman, Darby & Cushman ABSTRACT: A cellulose bleach tower in which at least two different bleaching steps are performed upon pulp flowing continuously therethrough, is provided with a set of concentric cylindrical screens through which a liquid containing used bleaching chemicals is drawn offthe pulp at the end of the first bleaching step. A liquid containing the bleaching chemical of the second step is spread by nozzles moving between or laterally of the screens. After the pulp has passed the screens and before the main part of the second bleaching step has taken place, the concentration of the bleaching chemical is equalized over the cross section of the tower by means of rotating scrapers which in a short zone of the tower displace the pulp back and forth in the transverse direction of the tower, whereas the pulp else flows truly axially of the tower.

PATENTEB mm mm 3, 599 .449

SHEET 1 0F 3 Fig,

"" INVENTOR Johan C C-- Rid- BY cmm bm Mm ATTORNEYS PATENTED ms! 7 I9?! SHEET 2 0? 3 INVENTOR ATTORNEYS PATENTEU mm 7 Ian SHEEF 3 BF 3 INVENTOR Johan Q-F Rih+e BY B05674 Cushmam ATTORNEYS CELLULOSE BLEACH TOWER WITH MEANS FOR SPREADING A BLEACHING AGENT THEREIN The invention relates to an upright cellulose bleach tower of the kind in which there is added to pulp flowing axially through the tower a bleaching agent which is spread in the pulp and distributed over concentric cylindrical faces situated at the same level as a series of concentric screening faces for screening off at least part of the liquid content of the pulp, spreading nozzles being arranged upon hollow arms attached to and supplying the bleaching agent from a tubular rotating shaft extending centrally in the tower. In a bleach tower of said kind which is known e.g. from the US. Pat. No. 3,348,390, the bleaching agent displaces the previous liquid content of the pulp, so that the consistency of the pulp is maintained substantially constant, but the distribution of the bleaching agent is uneven. This is of no great importance when the said spreading nozzles and screening faces are located close to the outlet end of a tower, from which the pulp is transferred to another tower wherein the treatment with the added bleaching agent takes place, because the transfer of pulp to said tower is almost unavoidably connected with stirring movements whereby the concentration of the bleaching agent within the pulp is equalized.

The invention relates to the case in which the treatment of the pulp with the bleaching agent supplied to the tower takes place in a zone of said same tower, and the object of the invention is to equalize the differences of concentration of the chemicals in the moving pulp column evolved from the concentric arrangement of the spreading faces and the screening faces. According to the invention said object is reached by providing said shaft with scrapers or similar means adapted for moving pulp inwardly towards the axis of the tower as well as outwardly towards its shell in order to distribute the bleaching agent evenly over the entire cross section of the tower in a layer of the pulp which, related to the axial direction of flow of the pulp, is situated closely after said concentric screening faces and before a zone of the tower wherein the main treatment by the bleaching agent takes place. In the treatment zone as well as in the zone where the bleaching agent is supplied and displaces another liquid radially out of the pulp and into the screens, the pulp moves as a plug, i.e. with a movement in the longitudinal direction of the tower which is uniform over its cross section, but according to the invention there is interposed between said two zones a zone or a layer of a restricted axial extension, wherein the pulp is caused to move with a superimposed horizontal component, i.e. transversely to the general feed direction of the pulp. On account of pulp being moved in said zone not merely radially outwards but also radially inwards by means of scrapers, strictly vertical flow of the pulp is secured in the treatment zone following said mixing or agitating zone, i.e. the pulp will not be fed more rapidly in certain parts of the tower cross section, e.g. along the tower shell, as would be the case if all scrapers were made to move the pulp away from the axis of the tower. Preferably the inwardly operating scrapers are located at another level than the outwardly operating scrapers, so that the mixing zone is composed of a layer with predominantly outward radial pulp movement and a layer of predominantly inward radial movement. This involves good equalization over the entire cross section of the tower and not merely locally restricted whirling movements,

The invention will now be more closely described with reference to the accompanying drawings.

FIG. 1 diagrammatically shows a bleaching plant in which five bleaching steps are performed in two bleach towers designed according to the invention.

FIG. 2 is a vertical sectional view at a larger scale, of a middle part of one of the towers, and

FIG. 3 is a corresponding sectional view of the upper end part of the same tower.

The bleach plant shown in FIG. 1 mainly consists of two upright cylindrical towers 11 and 13 through which cellulosic pulp is continuously passed while being bleached in a series of different bleach steps performed with different chemicals, during different treating time periods and possibly at different pulp consistencies. In the embodiment shown, pulp is introduced into the bottom end of the first tower 111, during the upward movement of the pulp a first treatment with chlorine takes place in zone C, then alkali extraction takes place in zone E thereafter treatment with chlorine dioxide follows in zone D and finally the first part of an alkali extraction step takes place in zone IE at the top of the tower. From there the pulp is fed through a conduit 15 to the top of the second tower 13 with downward pulp flow, wherein first the main part of the alkali extraction step E takes place, then treatment with chlorine dioxide is perfonned in zone D and finally the pulp is discharged at the bottom of the tower. The treatment dura tions of the various steps may be: for C, 1 hour, E l/2 hour, D,, 3 hours, E 1/2 hour and D 4 hours. The consistency of the pulp is maintained comparatively high in order to save tower space and secure plug flow. In the chlorine Step C the pulp may contain 8 to 10 percent dry fiber, whereas in the following steps there is preferably maintained a constant consistency of approximately 10 percent.

The details of the plant will now be :more closely described. Pulp supplied through a conduit 17 is pumped by a pump 19 through a flowmeter 21 and a mixer 23 into the center of the conical bottom 25 of the tower 11. Chlorine obtained from the conduit 27 is mixed into the pulp in the mixer 23. Through the entire length of the tower there extends a central tube 29 which is rotary journaled at its ends and is turned by a motor 31 with a gear 33. Said central tube encloses a number of conduits and serves to turn various means attached thereto as will be more closely described hereinbelow.

Attached to the central tube approximately at the level of the joint between the conical bottom 25 and the cylindrical shell of the tower is a screening device consisting of three sector-shaped plates 35 leaving between them sector-shaped apertures for the passage of the pulp. The cavity between perforated upper and lower walls of said plates communicates through the central tube 29 with an outlet 37. Liquid screened off the pulp by said screening device and departing through the outlet 37 is recirculated. The details of the screening device may correspond to the embodiment shown in the US. Pat. No. 3,298,900. As an alternative, the admixture of chlorine may take place in the manner described in said patent specification.

The pulp is concentrated to a dry content of about 8 to 10 percent on account of the described removal of part of its liquid content when it moves past the screening plates 35. Therefore, the main part of the chlorine bleaching operation takes place at said consistency while the pulp moves up through zone C.

Inserted between the zones C and E is a device for withdrawal from the pulp of the reaction products formed during the chlorine treatment and for spreading therein of new chemicals, viz alkali. Said device generally designated by reference numeral 39 and shown at an enlarged scale in FIG. 2, comprises a set of screens which is axially somewhat displaceable, and a rotary liquid spreader. The screen set consists of concentric cylindrical screen rings 41 attached to radial carrying arms 43 which extend through the tower shell and are attached to an exterior hydraulic operating device 45 by means of which the screen set, which during the screening operation follows the pulp upwardly in its axial movement, is periodically returned to a lower limit. The screen rings 41 and the arms 43 have inner cavities through which liquid screened offthe pulp flows to an exterior discharge conduit 47.

The rotary liquid spreader comprises horizontally and radi ally extending arms 49 carried by the central tube 29 and located at the same level immediately above the uppermost part of the screen set. Vertical arms 51 extend downwardly from said first-mentioned arms 49. When moved said arms 51 describe concentric cylindrical faces situated outside of and between the screen rings 41. Provided at the lower ends of the vertical arms 51 are nozzles for spreading liquid supplied through cavities in the arms 51 and 49 and through conduits 53 in the central tube 29 from a distributor tank 55 at the top of the tower. Spreading nozzles are provided also in the wall of the central tube. The nozzles 57, 59, 61 which are situated at the same level communicate each through a separate conduit with a pertaining compartment 63 in the tank 55 (see FIG. 3) and receive shares, controlled by adjustable weirs, of the liquid supplied to an outer annular chamber 67 by the conduit 65. Similarly the nozzles 69, 71, 73 situated at a level which is higher than that of the first-mentioned nozzles, communicate each through a separate conduit with a pertaining one of three other compartments 75 in the distributing tank 55, said compartments being filled with adjustable shares of the liquid supplied to a central chamber 77 by the conduit 79. The liquid supplied by conduit 79 and spread by the upper nozzles consists of a solution of NaOI-I, and the liquid supplied by conduit 65 and spread by the lower nozzles may consist of water or an alkalic effluent. The liquids ejected from the spreading nozzles move in essentially radial directions and displace the previous liquid content of the pulp, which contains chlorine and reaction products, so as to discharge the same through the screen rings 41 and the outlet 47. When the pulp reaches the level of the upper edges of the screen rings, the liquid therein has been completely replaced and now consists of alkali solution. However, any considerable amount of alkali should not escape through the outlet 47 and therefore the liquid spread through the upper nozzles is so proportioned that it does not reach the opposite screen faces. Consequently, at the level immediately above the screen set, pulp having passed adjacent to the screen faces will contain such weaker liquid as has been supplied through the lower spreading nozzles. Therefore, in respect of alkali concentration the pulp column will show a concentric pattern somewhat like the annual rings of a tree trunk. On account of the pulp moving as a solid plug,'said concentration differences would remain in the entire zone E and cause uneven chemical treatment; however, in order to eliminate said drawback, according to the invention the described impregnation with chemicals by displacement is combined with mechanical equalization caused by radial pulp movements in a layer situated close to the screen set but not so close as to disturb the displacement operation which requires a truly axial movement of the pulp. In the known method of mixing chemicals into pulp by agitation, e.g. in a mixer, it is easy to get a good equalization of the concentration but the pulp is diluted. In contrast thereto, according to the present invention the consistency of the pulp may remain essentially unchanged, approximately as much liquid being screened off as is supplied through the rotary nozzles, and a high degree of equalization of the chemicals in the pulp is obtained by moderate agitating movements inthe pulp on account of the chemicals being distributed in the pulp already by the displacement, so that the agitation merely serves to effect a final higher degree of even distribution of the chemicals. In addition thereto, it is possible by the displacement operation to remove from the pulp its previous content of chemicals simultaneously with new chemicals being spread in the pulp.

In the shown embodiment the radial equalization movements of the pulp are caused by scraper blades attached to the horizontal arms 49. The scraper blades are set obliquely to the radius in order to displace the pulp substantially radially when the arms 49 are turned, although a certain simultaneous movement of the pulp in the peripheral direction usually takes place, too. Some of the scraper blades are adapted to move the pulp radially inwards towards the center and other blades are adapted to move the pulp outwards towards the shell of the tower. Each arm 49 carries a plurality of scraper blades, viz lower scraper blades 81 and upper scraper blades 83. The lower scraper blades 81 are set obliquely at the same angle, and the upper scraper blades are also set obliquely at the same angle to the arm, but to the other side thereof. Furthermore,

the scraper blades are placed at different radial distances and with even intervals. Upon each arm the scraper blades are located alternately on the upper and the lower side. When the arms 49 are turned, the lower scraper blades will move the pulp outwardly towards the shell of the tower, whereas the upper scraper blades will return pulp in the direction towards the center of the tower, as indicated by arrows. In the short axial zone of the tower where the scraper blades are active, the slowly and continuously upward rising pulp therefore is subjected to a superimposed movement with horizontal components. In a lower layer of said zone the horizontal movement is directed predominantly outwards, and in an upper layer the movement is directed predominantly inwards, although, of course, movements in other directions may be present to a restricted extent in each of said layers as well as between the layers. The described arrangement of the scraper blades provides a pattern of pulp movements comprising loops and whirls with a considerable extension in the radial direction. This favors the spreading of pulp rising approximately straight above the screen rings 41 and having a low chemical concentration, and its mixing with the rest of the pulp. The scrapers causing an equally great displacement of the pulp in the inward as in the outward direction, the movement of the pulp in the alkali treatment zone B, is unaffected, but the pulp will rise there with a movement evenly distributed over the cross section of the tower, i.e. it will move as a solid plug.

Inserted in the tower 11 between the zones E and D, is a device 85 for screening off the reaction products obtained by the alkali treatment and for supplying chlorine dioxide to the pulp. Said device 85 is identical to the device 39. The liquid screened off departs through the outlet 87, and washing liquid and bleaching chemicals are supplied from a distributor tank 89 on top of the tower.

A third screening and spreading device 91 is located at the top of the tower l1 and serves to screen off and convey to the outlet 93 such reaction products as are obtained during the chlorine dioxide treatment and to introduce into the pulp, alkali supplied from a distributor tank 95. The main part of the treatment step E takes place in the top of the second tower l3, and therefore the device 91 differs from the devices 39 and 85 in that the horizontal spreader arms 97 carry merely a single series of equally set scraper blades 99, the object of which is to discharge pulp into the peripheral flume 101 and the connected conduit 15. The spreader device proper is located in the top of the tower 13 and comprises two series of scraper blades acting in opposite radial directions. These are attached to arms 103 carried by the rotary central tube 107 in the tower 13, which tube is turned by a motor 105, and are located at such a level that they operate in the pulp closely below its surface at the top of the tower 13, which surface is maintained constant by level control.

Inserted in the tower 13 between the zones E and D is a device 109 of the same construction as the above described device 39, but turned upside down compared thereto, in order to adapt the same to the flow direction of the pulp which in the tower 13 is downward and opposite to the flow direction of the pulp in the tower 11. Liquid carrying reaction products of the alkali treatments is withdrawn from the fibers by means of the screens 11] and is discharged through the conduit 113. Wash water and a new bleaching agent (chlorine dioxide) are supplied through the conduits IIS and 117, respectively, and are distributed in the container 119 between conduits extending through the central tube 107 and through cavities in the radial arms 121 and the vertical arms 123, to nozzles upon the latter through which the wash water and the bleaching agent are spread in the pulp at the level of the screens 111. The arms 121 which carry upper and lower scraper blades 125, 127, are in this case placed at a level situated somewhat below the lowermost parts of'the screens I l l.

' Near the bottom of thc tower,l3 there is a further screening and spreading device 129 of substantially the same design as the device 109. The reaction products obtained by the bleaching in zone D are screened off and discharged through the outlet conduit 131, and as a replacement thereof such liquid is spread in the pulp as is obtained from the conduits 133 and 135 and distributed in the container 137 upon conduits extending to the nozzles of the .spreading device. The supplied liquids may consist of washing water of two different degrees of purity, or possibly SO -Water in the lower nozzles. Said liquids are supplied through cavities in the arms 139 of the bottom scraper by means of which the bleached pulp is forwarded towards a central outlet in the bottom of the tower. In order to facilitate the discharge of the pulp a further quantity of diluting water may be supplied through stationary spray nozzles inserted into the tower shell and connected to the conduit 141.

The above-described bleaching plant is merely an example and it is obvious that the invention may be applied to bleach plants in which other than the described chemicals are used in the various bleaching steps and in which the division of the bleaching steps upon two or more towers differs from that shown.

What I claim is:

l. A multistage upright cellulose bleach tower of the kind in which there is added to pulp flowing axially through the tower at least two bleaching agents which are spread in the pulp and distributed over concentric cylindrical faces situated at the same level as a series of concentric screening faces for screen ing off at least part of the liquid content of the pulp, said tower including a lower series of concentric screening faces and an upper series of concentric screening faces defining ends of separate bleaching stages, spreading nozzles for supplying a different bleaching agent to each of said stages, said nozzles being mounted on upper and lower horizontally extending hol low arms positioned adjacent said upper and lower faces for supplying bleaching agent thereto and a tubular rotatable shaft extending centrally through and beyond said tower and containing therein at least a first and a second conduit, said upper and lower arms being attached to said first and second conduit means, respectively, and first and second reservoir means which contain different means being mounted upon the portion of said tubular shaft which extends beyond said tower and said first and second reservoir means communicating with said first and second conduit means respectively, for supplying different bleaching agents to said upper and lower arms, said arms being provided with scraper blades for moving pulp inwardly towards the axis of the tower as well as outwardly towards its shell in order to distribute the bleaching agents evenly over the entire cross section of the tower in a layer of the pulp which, related to the axial direction of flow of the pulp, is situated closely after the concentric screening faces of each series and before zones of the tower wherein the main treatment by the bleaching agent takes place.

2. A bleach tower according to claim 1, characterized in that a first set of scraper blades attached to said arms obliquely thereto so as to move pulp radially inwards, is located at another level than a second set of scraper blades also attached to said arms obliquely thereto but in the opposite sense so as to move pulp radially outwards.

3. A bleach tower according to claim 2, characterized in that each horizontal arm carries upper scraper blades as well as lower scraper blades, the scraper blades taken in a radial order being alternately upper and lower scraper blades.

4. A bleach tower according to claim 3, characterized in that one and the same tubular shaft extending through the entire length of the tower carries a plurality of sets of horizontal arms, each set located at a separate level and the arms thereof carrying upper and lower scraper blades, 

2. A bleach tower according to claim 1, characterized in that a first set of scraper blades attached to said arms obliquely thereto so as to move pulp radially inwards, is located at another level than a second set of scraper blades also attached to said arms obliquely thereto but in the opposite sense so as to move pulp radially outwards.
 3. A bleach tower according to claim 2, characterized in that each horizontal arm carries upper scraper blades as well as lower scraper blades, the scraper blades taken in a radial order being alternately upper and lower scraper blades.
 4. A bleach tower according to claim 3, characterized in that one and the same tubular shaft extending through the entire length of the tower carries a plurality of sets of horizontal arms, each set located at a separate level and the arms thereof carrying upper and lower scraper blades. 